Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations in a fuel vapor canister, and then purge the stored vapors during a subsequent engine operation. The stored vapors may be routed to engine intake for combustion, further improving fuel economy.
In a typical canister purge operation, a canister purge valve coupled between the engine intake and the fuel canister is opened, allowing for intake manifold vacuum to be applied to the fuel canister. Simultaneously, a canister vent valve coupled between the fuel canister and atmosphere is opened, allowing for fresh air to enter the canister. This configuration facilitates desorption of stored fuel vapors from the adsorbent material in the canister, regenerating the adsorbent material for further fuel vapor adsorption.
However, fresh air flow within and through the canister is not uniform. Regions of adsorbent that see relatively less air flow will retain relatively more adsorbed hydrocarbons. Typically, 10-15% of the canister will retain some quantity of hydrocarbons following a purge operation, and this amount may increase as the canister ages. The residual hydrocarbons may desorb over a diurnal cycle, leading to an increase in bleed emissions. Strategies to limit these bleed emissions have included secondary canisters and heating elements, both of which increase manufacturing costs and require additional diagnostic testing.
Furthermore, current and future engine systems may be configured to operate under relatively low manifold vacuum conditions. While this may increase engine efficiency, it also reduces the opportunities for fuel vapor canister purging. This may particularly apply to hybrid vehicles, which have a limited engine run time to begin with. For example, limited engine run time in hybrid electric vehicles (HEVs) may limit engine manifold vacuum, thus decreasing opportunities for canister purging operations. Even if purge conditions are met, the conditions may only be held for a short period of time, leading to incomplete purge cycles. This may result in residual fuel vapors stored in the canister for long periods of time. Typically, the canister is coupled to atmosphere while the vehicle is off. Over the course of a diurnal cycle, the fuel vapors may desorb from the canister as temperature rises, resulting in an increase in bleed emissions. The canister vent valve could be maintained closed, but the vent valve is typically a solenoid valve requiring constant power to stay shut, which could drain the battery if the vehicle is left off for a significant period of time.
The canister purge valve is typically located at the intake manifold. Positioning the canister purge valve at the intake manifold serves to limit hydrocarbon transport delay between the canister purge valve and engine cylinders. For example, if a canister is loaded with fuel vapors and the canister purge valve were located away from the intake manifold near the canister, by the time exhaust gas sensors indicate that canister load is high, a stall condition may occur due to rich air/fuel mixture in the intake. As such, it is desirable to have the canister purge valve as close to the cylinders as possible (i.e. mounted onto the intake manifold) to prevent engine drivability issues such as hesitation, surge, or even stall. However, with the canister purge valve mounted at the intake, a significant pressure drop is observed in the vapor line between the intake manifold and the canister as a result of duty cycling the canister purge valve. Significantly, the vapor line between the canister and the canister purge valve can be as long as 8-10 feet in some vehicles. In such cases, the pressure drop results in a considerably reduced vacuum as seen at the canister, decreasing the ability of engine vacuum to thoroughly purge hydrocarbons from the canister. Systems and methods enabling precise control over purge flow rates when only low flow rates are desired, while additionally enabling sufficient vacuum to thoroughly purge the fuel vapor canister, would improve engine operations and reduce evaporative emissions.
US patent application U.S. Pat. No. 5,115,785 teaches a pulse width modulated solenoid-actuated canister purge valve and a vacuum-actuated canister purge valve arranged in parallel between engine intake and the fuel vapor canister. Below a certain duty cycle of the solenoid-actuated valve, only the solenoid-actuated valve is open. At higher duty cycles, both flow paths are open, resulting from the increased vacuum at higher duty cycles opening the vacuum-activated canister purge valve. As such, the system purports to enable control at low flow rates, but purports to be capable of higher flow rates. However, the inventors herein have recognized potential issues with such systems. As one example, responsive to vacuum above a threshold, the vacuum-actuated valve is always open, without the potential to be duty-cycled. In other words, duty cycling the solenoid-actuated valve, even at high rates where the vacuum-actuated valve is open, may still result in significant pressure drop between the intake manifold and the canister, thus decreasing the ability to effectively clean the canister.
US patent application U.S. Pat. No. 6,202,632 teaches a first controllable purge valve and a second controllable purge valve, arranged in parallel between an intake manifold and a fuel vapor canister. The first valve has a smaller flow cross-section than the second valve. As such, the first valve may be first opened (i.e. duty-cycled) for purging of the canister, and if greater flow is desired, the second valve may subsequently be opened. However, the inventors herein have recognized potential issues with such systems. As one example, U.S. Pat. No. 6,202,632 teaches that the first valve is controlled via a pulse-width modulated signal, whereas the second valve may only be directed to an open or closed position. As such, while greater flow may be attained with the second valve opened, the flow (and pressure drop) between the canister and intake manifold are strictly dependent on the level of intake manifold vacuum and the distance between the intake manifold and the canister. Accordingly, in some examples significant pressure drop may still be observed, thus reducing the ability to effectively clean the canister.
Thus, the inventors herein have developed systems and methods to at least partially address the above issues. In one example a method is provided, comprising during purging fuel vapors stored in a fuel vapor canister to an intake manifold of an engine, first duty cycling a first canister purge valve and commanding open a second canister purge valve, and then stopping duty cycling the first canister purge valve, opening the first canister purge valve, and commencing duty cycling the second canister purge valve responsive to a canister load below a threshold canister load.
As one example, opening the first canister purge valve and duty cycling the second canister purge valve increases air flow through the fuel vapor canister as compared to duty cycling the first canister purge valve and opening the second canister purge valve. In this way, increased air flow to the fuel vapor canister may be provided under circumstances where it is indicated that canister load is below a threshold level, thus reducing potential engine stall conditions resulting from the introduction of fuel vapors into an engine under increased air flow. In one example, the first canister purge valve is coupled to the intake manifold and the second valve is coupled to the fuel vapor canister, and the first and second canister purge valves are arranged in series. By operating the first and second canister purge valves to selectively increase air flow to the fuel vapor canister, thorough cleaning of the fuel vapor canister may be accomplished during purging events, and may thus lead to reduced evaporative emissions, and prolonged fuel vapor canister lifetime.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.